Path: utzoo!utgpu!water!watmath!clyde!att!rutgers!rochester!pt.cs.cmu.edu!sei!sei.cmu.edu!firth
From: firth@sei.cmu.edu (Robert Firth)
Newsgroups: comp.arch
Subject: Are all RISCs the same?
Message-ID: <6903@aw.sei.cmu.edu>
Date: 6 Sep 88 17:33:44 GMT
References: <58@zeno.MN.ORG>
Sender: netnews@sei.cmu.edu
Reply-To: firth@bd.sei.cmu.edu (Robert Firth)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
Lines: 35

In article <58@zeno.MN.ORG> gene@zeno.UUCP (Gene H. Olson) writes:

>*	The Motorola, Intel, MIPS, SPARC, HP, and IBM RISC
>	architectures are incredibly similar.  In their basic
>	instruction sets, none of them has any significant
>	advantages over the other.

Sorry, Gene, I'm going to disagree with your first point, and hence
with your conclusions.  These machines differ in many respects.

(a) Some have register window systems.  This is a disastrous design
    error that will ultimately doom them.  In particular, the greatly
    increased context-switch time, and the unpredictability in the
    cost of a simple procedure call, make register-window machines
    unsuitable for hard real time applications.

(b) Some have elaborate and expensive non-RISC features.  One machine
    in your above list has ADDRESS MODE computation times that can take
    from 1 to 5 cycles, good grief.  Some have those good old "high level
    language support" instructions that 20 years' experience have proved
    a total loss.

(c) Some have imprecise exception states that make both true recovery
    semantics and true continuation ('Ada-like') semantics almost impossible
    to realise.  (One gives you precise exception states if you slow the
    machine down by about 2.5, I believe)

(d) Some come with manufacturer-designed procedure calling sequences that
    are wired into virtually all the system software and hence almost
    inescapable.  They are also gruesomly inefficient.

And, of course, I do believe some of the above machines have significant
technical advantages over the others.  No prizes for guessing which. But
we should also remember that technical excellence is not the only thing
determining success!